Re: [PATCH 2/2] x86/amd_node: Avoid divide by zero on virtualized systems

[David Laight <david.laight.linux@gmail.com>]

On Mon, 10 Aug 2026 10:47:06 -0400
Yazen Ghannam <yazen.ghannam@amd.com> wrote:

> On Thu, Aug 06, 2026 at 12:01:57PM -0400, Jason Andryuk wrote:
> > On a virtualized system, the number of nodes does not have a
> > relationship to the number of roots.  A Xen PVH dom0 can calculate
> > roots_per_node as 0, which crashes with a divide by zero in:
> > 
> >     if (count++ % roots_per_node)
> > 
> > On a virtualized system, default the value to 1.  The issue is seen with
> > Xen, but it could affect other systems.
> > 
> > Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>  
> 
> I agree with the idea with some minor feedback below.
> 
> > ---
> > X86_FEATURE_XENPV is only for PV, but this is observed with a PVH dom0.
> > ---
> >  arch/x86/kernel/amd_node.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
> > index ea553267e5fa..c5025e5291b6 100644
> > --- a/arch/x86/kernel/amd_node.c
> > +++ b/arch/x86/kernel/amd_node.c
> > @@ -286,6 +286,11 @@ static int __init amd_smn_init(void)
> >  		return -ENOMEM;
> >  
> >  	roots_per_node = num_roots / num_nodes;
> > +	if (roots_per_node == 0) {  
> 
> Can be '!roots_per_node'.
> 
> > +		if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
> > +			pr_err(FW_BUG "Error detecting roots per node.\n");
> > +		roots_per_node = 1;
> > +	}  
> 
> I don't think this is necessarily a Firmware bug.
> 
> Likewise, it can be a 'warning' rather than 'error'. I interpret 'error'
> as something known to be incorrect. This was my position before on this
> topic. But the various contrary reports changed my mind (even if they
> come from virtualization).
> 
> Furthermore, I think 'warning' is more appropriate. It says "We found
> something unexpected. We're letting you (the user) know about it. And
> we'll mitigate it to avoid an error."
> 
> Anyways, I just wanted to write out some thoughts since this topic has
> come up a few times.
> 
> Possible rework for the above change:
> 
> 	if (!roots_per_node && !cpu_feature_enabled(X86_FEATURE_HYPERVISOR)
> 		pr_warn("Expected at least 1 root per AMD node.\n");
> 
> 	roots_per_node = max(roots_per_node, 1);

I'd not add the max(), just;
	if (!roots_per_node) {
		roots_per_node = 1;
		if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR)
			pr_warn("Expected at least 1 root per AMD node.\n");
	}

David

> 
> In any case, we should just go with a simple fix for the virt cases.
> 
> I think we could even do away with caching the 'root' devices. But
> that'll be another rework/cleanup.
> 
> Thanks,
> Yazen
>